When the lateral dimension of the tool head is close to or greater than a quarter of the longitudinal wave length, the tool head will produce severe lateral vibration. The coupling of the lateral vibration and the longitudinal vibration makes the amplitude distribution of the tool head's radiation surface uneven, which seriously affects the welding quality. To solve the problem of uneven amplitude distribution of the two-dimensional tool head's radiating surface, in the paper we conduct an optimized design study on a two-dimensional ultrasonic plastic welding system. First, using the theory of phononic crystal dislocations, we construct a nearly periodic phononic crystal homogenous dislocation junction on a large-sized long strip tool head, and use the homogenous dislocation junction to change the regular lattice arrangement of the phononic crystal structure to adjust the position of the band gap and increase the width of the band gap, so that the operating frequency of the two-dimensional ultrasonic plastic welding system can be located in the band gap of the lateral vibration of the tool head, and the effective control of the lateral coupling vibration of the tool head can be achieved, thus optimizing the amplitude uniformity of the radiating surface of the tool head and increasing the amplitude gain. Although the homogenous dislocation junction structure improves the amplitude uniformity of the radiating surface of the tool head, the lateral dislocation effect of the homogenous dislocation junction causes the sound waves in the band gap frequency range to propagate along the dislocation channel, while the dislocation line channel is located in the middle of the tool head, which results in a larger displacement of the middle part of the tool head's radiating surface, and a smaller displacement on both sides. Therefore, the further optimizing of the two-dimensional tool head is required. In this study, the nearly periodic phononic crystal inclined groove structure is used to better optimize the amplitude distribution uniformity of the radiating surface, and the influence of the inclined groove structure parameters on the longitudinal resonance frequency and amplitude distribution uniformity of the ultrasonic plastic welding system are analyzed, that is, the inclined groove can better improve the uniformity of the amplitude distribution than the straight groove, but the angle of inclination of neither the inner nor outer inclined grooves should be too large: the optimal range is 3 degrees-6 degrees. In addition, the difference in inclination angle between the inner inclined groove and the outer inclined groove should not be too large, and the angle difference from 0 degrees to 2 degrees is best. The simulation results show that the nearly periodic phononic crystal homogenous dislocation junction and inclined groove structure can optimize the two-dimensional ultrasonic plastic welding system, which provides a basis for further research on the theory of lateral vibration suppression.
